The present disclosure relates to a method for controlling a printing operation, a computing unit for carrying out the method, a register mark sensor, and a control system for a printing press.
During multicolor printing, for example in rotary printing presses, the application of the individual color separations, in particular for cyan, magenta, yellow and black, takes place in successive printing units. The printing material is provided as roll material and led endlessly through the print unit. Critical for the printing quality achieved is that the printed images of the individual colors lie exactly above one another. The superimposition of the printed images is designated register. For the purpose of mutual alignment of the individual printing units, in addition to the actual printed image, register marks, for example in the form of register crosses, triangles and so on, are printed on by each printing unit. By using these marks, an offset between the individual printed images can be detected online by an optical measuring system. In rotary printing systems, this measuring system is generally a constituent part of a control system, what is known as register control. The register control intervenes in the printing process via suitable actuating elements and compensates for register deviations recognized by the optical measuring system (detector; register mark sensor). In particular, the actuating elements are able to change the web length of the printing material between successive printing units in such a way that the printed images of successive print units lie above one another. Detectors used are contrast sensors, color sensors or cameras, which typically operate by reflection. The material web is illuminated constantly or in a pulsed manner by using a suitable light source (white or colored light), and the reflected light is detected by the sensor and evaluated.
Before any sensing by contrast or color sensors, a teaching operation is normally carried out, in which a signal threshold is learned, with the aid of which a distinction is drawn between “mark” and “no mark”. The time of that scanning in which the signal to be evaluated exceeds or falls below the signal threshold is taken as the time of the contrast change. DE 10 2008 049 908 A1 discloses a color-sensitive register mark sensor which, for the purpose of improved recognition of the register mark position, carries out an evaluation of at least two items of color information. It is also possible for low-contrast register marks to be recognized reliably. Color control is not treated.
In addition, color control or color checking is carried out during printing. Nowadays, this is increasingly carried out as in-line color control, as it is known, during the printing operation, color measuring areas (color control strips) being detected by means of specific color sensors. In-line color control is described, for example, in DE 10 2004 021 599 A1.
The disadvantage with the prior art is that relatively many and also complicated detectors are needed for the register control and the color control. Furthermore, in addition to the register marks, the color measuring areas also have to be printed on, which increases the requirement for printing ink, which in specific cases is relatively expensive.
There is therefore a need to devise an improved method for controlling a printing operation which requires less effort.